Tree-Ring Dating
(Dendrochronology)

Dr Martin
Bridge

Institute of Archaeology, University College London
For further information, see Martin's website at: www.martinbridge.co.uk

Background

Just
about everyone is familiar with the idea that trees put on one ring a year, and that therefore
you can tell the age of a tree by counting its rings. Almost everyone has heard of radiocarbon
dating too - the technique that has revolutionised much of the dating framework of archaeology.
Few realize however that radiocarbon dates are actually calibrated using dated tree-ring
series, and that they give a range of years, sometimes quite a wide range, in which the item
was living.

The
stunning and, to me, still exciting thing about tree-ring dating is that it is capable of
determining the actual year of growth of a particular ring. When complete sapwood (the outer
living rings in a growing tree) is found on an historic timber, it is possible to determine
the season of the calendar year in which the tree was felled. Since throughout history until
comparatively recently, trees were used 'green', that is unseasoned, if one determines when
trees were felled, one is usually within a year or two of when they were actually used.

In
fact, the idea that trees lay down a ring each year is an over-simplification; in different
parts of the world trees do not necessarily lay down a ring on a yearly basis, and some trees
in unusual conditions will miss rings, or produce multiple rings in a year - but we needn't
get caught up in this here!

The
variation in the ring widths from year-to-year reflect the different rates of growth which
tell the story of each tree's history. If grown in a hedgerow, with little competition from
other large trees, the tree may grow quickly from the start. In a woodland the tree may grow
very slowly at first until it reaches the canopy and is well established. Storms, pests,
diseases etc. may all leave their mark in the tree's ring- width pattern. Nevertheless, the
one factor that influences the growth of all the trees of the same species over quite a wide
geographical area will be the weather during the growing season. There is generally therefore,
a good deal of similarity between large parts of the ring-width patterns between all trees
of the same species growing at the same time in a region.

This
fact forms the basis of dendrochronology. In some parts of the world the relationship between
the weather and tree growth is relatively simple. In the arid south-western United States
for example, a wet year produces a wide ring and a dry year a narrow ring. This fact was
noted at the start of last century and it was soon realized that long patterns of ring widths
were like a fingerprint, unique to the time in which the tree grew. If one had enough rings
available to be sure of matching the patterns, the time when the tree grew could be determined.
After a great deal of work, led by Andrew Douglass, a long dated tree ring chronology became
established and it became possible to date the wood used in the pueblos, ancient ruins of
the native population. Similar work was being done in Continental Europe, particularly in
Germany, where the main species investigated was oak, used by the human population for thousands
of years as a major resource for building timbers and other artefacts.

Dendrochronology
in Britain

For
a long time it was assumed that the complex maritime climate experienced in Britain would
mean that we would not have these clear-cut patterns and that dating would not be possible.
Indeed, a group of German dendrochronologists looked at several grand oak structures and
declared in the early 1970s that any similar dating work in Britain would require an awful
lot of work.

Happily,
since then a number of dendrochronologists have been working away establishing the patterns
of growth for oak trees covering many centuries, indeed millennia, in various parts of the
British Isles. Starting with living oaks to give the exact dates to the outer years, one
can overlap the outer rings of trees felled earlier, say in Victorian times, with the inner
rings of the living trees to produce a continuous chronology going back perhaps three centuries
or more. Similarly, using progressively older sources of timber, the chronology becomes established.

When
a new, undated oak sample is found, the ring-width pattern is measured and compared with
the extensive database to see if its distinctive pattern closely matches the patterns derived
from dated series. From what has been said above, hopefully it is clear that the chances
of dating a single series is going to be quite small. Any given tree will have superimposed
on the basic climate response its own particular life story. If several contemporaneous timbers
are looked at however, say a dozen timbers all from the same phase of building, it is often
possible to match the patterns within the group of timbers and then derive a mean pattern
of growth. This has often been likened to a radio signal - one sample gives a 'noisy' signal,
difficult to distinguish, but several samples reinforce the underlying signal making it clearer
to read.

Of
course, if a single woodland has been used as the source of constructional timbers, and all
the trees in that wood have experienced similar management regimes, or all been subjected
to insect defoliation, flood or similar local disturbance, then the trees may not show the
typical regional growth trends, and may not match any other site. Thus it is not always possible
to date timbers which at first sight appear to be suitable, i.e. have lots of rings and show
variation in ring-width from year to year with no abnormal sudden changes of growth rate,
but the rates of success are getting better and better as the database gets larger and larger.

Size
of timber is not a particularly good guide to the age of a tree - and hence the number of
rings in a timber, and there is some variation across the country. A good-sized ceiling beam
in East Anglia may only contain 50-60 rings, whereas the same sized beam in the Welsh borderlands
might contain 150-160 rings. Small rafters or studs may contain more rings than larger structural
beams - it all depends on the conditions the tree was growing in. Most dendrochronologists
like to have a minimum of 50 rings before attempting to find a unique match. Here the fingerprint
analogy is useful again - a partial 'dab' may share several characteristics with a suspect's
full fingerprint, but in order to convict the suspect a minimum number of points of coincidence
must be found. With tree rings, one may get statistical matches between series, but unless
there are a good number of rings available, and the result can be replicated against several
independent datasets, the dendrochronologist will not consider the match to be good enough
to ascribe it a date. Therefore, with tree rings one should either get a reliable date, or
nothing at all! There is no vague indication, like 'it is probably fifteenth century', one
can either place the series to a particular run of calendar years, or one can't.

Many
building historians can give a reasonably accurate date range for a building based on the
style of carpentry employed - some features are characteristic of just a few decades in a
particular century, although many were employed for long periods and it may only be possible
to give quite wide date ranges. Many of these dates have been independently tested using
dendrochronology, and found to be quite reliable. There is a danger here of circularity however,
dating on stylistic grounds alone can be way out on odd occasions, often by more than a century!

What
is involved in getting a dendro-date?

The
preference for multiple contemporaneous samples has been explained above. Whilst it may be
possible to date a single timber, two things have to be kept in mind - firstly, the chances
of getting a date are far higher if multiple timbers are used, and secondly, how meaningful
is the date for a single timber? It may be a timber that is re-used, or had been stored before
use, or it may be an undetected repair. If several timbers give dates within a few years
of each other, one can have far more confidence that the whole structure has been securely
dated. Then of course there is the question of the use of re-used timbers - these are usually
apparent because of mortices and peg holes etc. that do not match with any configuration
of the building as it is today. It is rare for a large batch of timbers to be re-used as
the only construction timbers, so multiple samples should pick up the different ages, but
of course salvaged timbers have always been a valuable commodity, and dendrochronology can
only date when the tree was growing.

The
dendrochronologist would love to have end-grain cross-sections of timbers to see any abnormalities
around the tree, avoid knots etc. Of course this is rarely possible in building timbers,
unless ends are being sawn off in repair work etc. More commonly, the dendrochronologist
will want to take cores down the radius of what was the tree. This may mean going in at odd
angles - and they will also want to core through sapwood remains as this ensures that one
has reached the outermost rings of the tree. If no sapwood is available at all, it may be
possible to date the ring-width series, but these will only give an end date after which
the trees were felled.

Most
British dendrochronologists use 16mm diameter coring bits, leaving a hole about the size
(or slightly smaller) than most peg holes. The holes may be left open, or plugged with a
dowel, and the plug itself may be left clearly visible or disguised to be almost invisible.
There are several arguments for and against each of these possibilities - these need to be
discussed with regard to each study.

What
should I do if I want to get my house dated?

The
first thing to do is to take a look around for yourself. Whilst most timber-framed buildings
are of oak, there is a lot elm and other species used. Sadly at present it is only oak that
can be dated in most instances - although imported conifers may also be possible to date.
Not everyone can recognise the characteristics of different species of course - don't be
afraid to ask! Another good starting point may be to look at the guidelines on dendrochronology
produced by English Heritage and available free from the Scientific Dating Section, English
Heritage, 23 Savile Row, London W1X 1AB (020 7973 3000) - these give additional information
on the background to the subject, practicalities of sampling, possible need for permission
to core in Listed Buildings, help on interpreting any results produced etc. and list most
of the dendrochronology laboratories in the country, with their contact details.

You
may also be able to determine roughly how many rings there are in your timbers - try looking
at any end grain, perhaps in empty mortices or where timbers have shrunk in joints. If you
get your eye in, it is possible to see the rings side on in unpainted timbers. Also ask yourself
if you can see any sapwood - usually more fragile and often different in texture and colour.
Good places to look are in attics where the timbers are often less disturbed than elsewhere.
Don't worry if you cannot do any of these, the dendrochronologist will be able to assess
the potential of your timbers, but it may be a little expensive to get the professional to
come and look and to say that they are sorry but your timbers are not suitable!

The
problem of cost is difficult - it depends on so many things, location, access to timbers,
number of phases of building to be looked at etc. This is probably best discussed directly
when you first contact a dendrochronologist - but remember, a lot is involved, for every
day spent taking samples in the building there is probably another 3-5 full days spent preparing
and measuring samples, analysing the results and writing a suitable report. There are also
the questions of specialist equipment, experience and insurance which don't come cheap!

Some
people like to charge per sample - I personally do not like this system as it encourages
the person commissioning the work to have as few samples as possible taken, whereas the outline
given above should convince you that the chances of success, and the interpretation of the
results are both likely to benefit from a larger number of samples being taken. Naturally
there are questions of diminishing returns, aesthetic and structural considerations which
will limit the number of samples too.

A
final plea

Yes
dendrochronological dating can be expensive - but please, even if you cannot afford to consider
such things now, remember, it is far easier to have the dendrochronologist in when work is
being done on repair or renovation - it is so frustrating to hear that timbers where readily
accessible six months again when work was being done. Also - if timbers are being cut out
for some reason, and they have more than fifty or so rings in cross-section, do label their
origin and keep them safe for future possible study. Again, it is so frustrating to hear
that the ends of all the rafters, or the beam taken out when that room was changed, was burnt
or put in the skip etc. Just a small slice, thick enough to hold together (say 5cm or 2")
is all you need to keep for each timber. Once the timber has gone it has gone for ever!

About
the author

Dr
Martin Bridge has been involved in dendrochronology since starting his PhD studies in 1979.
He has worked in New Zealand, Ireland and Newfoundland, but most of his work has involved
dating historic timbers and studying tree rings in living trees in southern Britain and north-west
France. He has worked on the Mary Rose, and on trees felled at Kew Gardens. He is currently
Lecturer in Dendrochronology at the Institute of Archaeology, University College London.

Website: www.martinbridge.co.uk contains details
of his publications and county-based lists of the buildings he has dated.

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